Mechanisms of muscular hypertonia and weakness in spastic limbs will be studied using spastic-hemiparetic adult human subjects. Hypertonia is known to result from an abnormally responsive stretch reflex. The question to be addressed is whether this stretch reflex abnormality arises simply because stretch reflex threshold is reduced (a consequence of increased motoneuronal excitability), whether the inherent stiffness of the reflex response is also abnormally augmented or whether both abnormalities coexist. This distinction if fundamental for understanding the neurophysiological bases of spasticity and for designing techniques allowing quantification of this disorder. Stiffness of normal and spastic limbs will be compared at comparable initial force and joint angle using controlled variations in load force or in limb position as test perturbations. Should differences in stiffness emerge, the reflex contributions of Ia, II and Ib afferents will be assessed. Mechanisms of weakness in spastic muscles will be studied focusing on contributions provided by anomalously low rates of motor unit discharge. Spectral analysis of surface emg recordings will focus on differences in power at low frequencies, the portion of spectrum where rate contributions become evident. Recorded differences will be studied using single unit intramuscular recordings. The role of cutanious and small diameter afferents from muscle groups III and IV in mediating the disorganization of motor unit recruitment and rate modulation will be studied, by comparing the reflex resopnse of muscles to cutaneous nerve muscle stimulation on normal and spastic sides of the same hemiparetic-spastic subject. The studies on limb stiffness may provide techniques useful for the quantification of the severity of spasticity. Such techniques are necessary for accurate assessment of the response to various therapies (pharmacologic, surgical or physical) and for charting the natural history of neurologic illnesses producing spasticity. The studies on weakness may provide an assessment of the importance of segmental afferent imput in mediating the abnormal patterns of motor unit recruitment and rate modulation. Such segmental imputs may be amenable to therapeutic modification.